From Wikipedia, the free encyclopedia:
Sender Policy Framework (SPF) is an email validation system designed to prevent email spam by detecting email spoofing, a common vulnerability, by verifying sender IP addresses. SPF allows administrators to specify which hosts are allowed to send mail from a given domain by creating a specific SPF record (or TXT record) in the Domain Name System (DNS). Mail exchangers use the DNS to check that mail from a given domain is being sent by a host sanctioned by that domain's administrators.
I'm glad you found Zoho easy to set up. Unfortunately, it doesn't seem like there's any way for it to be integrated into Discourse. However, there is another free service called Mandrill (https://mandrill.com) that you can use (and is in fact, officially endorsed by Discourse). You can find their tutorial on setting it up to work with Discourse at https://github.com/discourse/discourse/blob/master/docs/INSTALL-email.md. Sorry to be redirecting you all over the place.
You'll also need to set up a Postfix server on your website host (I assume you have root/SSH access). Guide to configuring Postfix for use with Mandrill: http://help.mandrill.com/entries/23060367-Can-I-configure-Postfix-to-send-through-Mandrill-